1. Field of the Invention
This invention relates to zoom lenses, and more particularly to zoom lenses of high relative aperture but, nevertheless good optical performance throughout the entire zooming range, and which are suited to photographic cameras or video cameras.
2. Description of the Related Art
The photographic camera, video camera and like ones have had a trend of demanding compact zoom lenses of large relative aperture and high range with high optical performance.
Of these, the video camera, because of its image pickup element being relatively low in sensitivity, should be equipped with a zoom lens of as large an aperture ratio as possible.
At present, the 2/3 inch image pickup tube is widely used in the video cameras because of their compactness and image quality. Also, from points of view of improving the managability and facilitating minimization of the bulk and size, 8 mm video cameras coming into increasing use. For the image pickup tube to be used in them, a further advance in the minimization of the size is required without sacrificing the image quality. Presently, 1/2 inch tubes and 1/2 inch image pickup boards are being employed.
In the video camera using the 2/3 inch tube, the speed of the zoom lens has not necessarily been more than F/1.4. But, when the 1/2 inch tube is used, because the area of the image pickup surface is nearly halved as compared with the 2/3 inch tube, the required speed for the zoom lens must be not slower than F/1.2.
Such a high relative aperture zoom lens has been proposed in, for example, Japanese Laid-Open Patent Application No. Sho 60-51813 (USP No. 4,621,905).
In general, in the called 4-component zoom lens, to achieve a large aperture ratio of about F/1.2, it becomes necessary to set forth so proper lens forms for the zoom section and the image forming section arranged on the image side of the zoom section to be stationary during zooming so that the residual aberrations of the zooming section are well corrected.
Also, to achieve a shortening of the total length of the lens, a minimization of the size of the first lens component counting from the front is advantageous. Yet, it also becomes important to set forth a proper lens form for the image forming section.
However, if the minimization of the size of the entire lens system and the increase of the aperture ratio are based merely on the strengthening of the refractive power of each lens component, spherical aberration in the paraxial region, coma in the marginal zone and higher order aberrations such as sagittal halo will largely be produced. Hence, it will become difficult to obtain a high optical performance.
For example, the strengthening of the refractive power of the first lens component for the purpose of achieving the minimization of the size leads to increase of overall image magnification of the other components in the zoom section and the image forming section. As a result, the first lens component produces too many aberrations, and the tolerances of the design factors becomes severe. Hence, it becomes very difficult to obtain a desired optical performance.
As examples of the related art, there are Japanese Laid-Open Patent Application Nos. Sho 60-208723 and Sho 60-260912.